Bones may be reinforced with rigid structures, for example plates, having apertures through which bone screws may pass. Such reinforcing is useful for addressing fractures or other trauma, correcting alignment, or treating disease. A bone plate is used to span a fracture, or to join different bones. It is important that a bone screw securing a bone plate does not loosen and separate from the plate postoperatively. This separation or backing out is typically a result of the failure to achieve sufficient screw thread purchase in the bone, although stripping of the screws has also been known to cause this problem. Regardless of the cause of the hardware failures, a surgeon must repair or replace the broken parts, requiring an undesirable invasive procedure.
Devices for preventing this separation may be associated with the plate and/or screw. Examples include a cover positioned over all or part of the screw head, or an expansion-head screw.